This invention relates to the production of double-walled thermoplastic pipe. The invention provides for the production of such pipe, formed from concentric outer and inner pipes, in which the outer pipe is molded into the shape of a helical thread.
The invention finds particular application to the production of double-walled thermoplastic sewer pipe, in which the inner pipe is cylindrical and smooth-walled to provide for the passage of sewege effluent therethrough, with the outer pipe being corrugated for strength. The conventional way of making such pipe is by utilizing a series of split molds carried by continuous chains, the split molds coming together about the outer pipe to form corrugations therein. An example of this type of technology is given by the following U.S. patents:
______________________________________ U.S. Pat. No. Date Patentee ______________________________________ 3,924,992 9 December 1975 Hegler 3,976,414 24 August 1976 Hegler et al 3,751,541 7 August 1973 Hegler 3,677,676 18 July 1972 Hegler 3,538,209 3 November 1970 Hegler 3,994,644 30 November 1976 Hegler 3,776,679 4 December 1973 Hegler ______________________________________
The use of split molds involves elaborate machinery, and requires an exact matching of mold halves in order to avoid noticeable seams in the pipe and properly to form the corrugations.
It is also known to feed a strip of thermoplastic material on the outer surface of a thermoplastic pipe to form a helical thread thereon. See the following German publication: "Erfahrungen bein Herstellen von Grobrohren aus PVC hart", by Von Ing. H.-W. Selbach, Bad Oeynhausen.
This separate feeding of strip also involves cumbersome machinery, as is apparent from the publication.
In the present invention, a double-walled pipe of thermoplastic material is produced by forming an outer pipe over and concentric with an inner pipe. The two pipes are then passed through a mold which rotates about the longitudinal axis of the pipes and which acts upon the outer pipe so as to form it into the shape of a helical thread about the outer surface of the inner pipe. The helical thread is formed so that the root section thereof is bonded to the outer surface of the inner pipe, advantageously through the use of rotating rollers spaced about the outer pipe along a helical path. A vacuum is applied to the outer pipe through the rollers to aid in the formation of the helical thread. Preferably, the circumferential edges of the rollers are spaced different distances from the longitudinal axis of the pipes to enhance the thread forming operation. The use of a rotating thread-forming mold greatly facilitates the helical thread forming operation and is capable of being carried out with relatively simple machinery. The exact matching of mold halves and the providing of extensive production line facilities as in the Hegler patent approach is not required.
The use of rotating molds to provide helical corrugations is not, in and of itself, novel. Such rotating mold assemblies have, however, in the past been limited to use upon individual pipe. The use of such rotating mold assemblies has not been suggested for double-walled thermoplastic pipe. Examples of the use of rotating molds for corrugation forming purposes in single-walled pipe is given by the following patents:
______________________________________ U.S. Pat. No. Date Patentee ______________________________________ 3,529,047 15 September 1970 Yoshida et al 3,692,889 19 September 1972 Hetrich 4,017,244 12 April 1977 Vellani ______________________________________
The invention will be more completely understood by reference to the following detailed description.